Schedule 2 - Parent Company Financial Statements - Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 4.6	$ 37.9	$ 29.7	$ 13.5	$ 16.8	$ 85.7	$ 114.5
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Parent									(2.4)	(151.9)	259.7
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Parent									0.2	(25.4)	(25.1)
Unrealized Holding Gains (Losses)									(2.2)	(177.3)	234.6
Unrecognized Postretirement Benefit Costs Arising During the Year									14.2	3.0	(106.4)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax									1.0	0.0	0.0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax									6.3	23.1	7.9
Net Unrecognized Postretirement Benefit Costs									20.5	26.1	(98.5)
Foreign Currency Translation Adjustments									(0.3)	(1.4)	(1.0)
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustments									(0.3)	(1.4)	(1.0)
Gain on Cash Flow Hedge									1.6	0.0	0.0
Other Comprehensive Income (Loss) Before Income Taxes									19.6	(152.6)	135.1
Income Tax Benefit (Expense):
Securities Held by Parent									1.0	53.9	(91.6)
Securities Held by Parent									(0.1)	8.9	8.8
Unrealized Holding Gains and Losses									0.9	62.8	(82.8)
Unrecognized Postretirement Benefit Costs Arising During the Year									(5.0)	(1.1)	37.5
Curtailment Cost Recognized									(0.4)	0.0	0.0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax									2.3	8.0	2.8
Net Unrecognized Postretirement Benefit Costs									(7.3)	(9.1)	34.7
Foreign Currency Translation Adjustments									0.1	0.5	0.4
Reclassification Adjustment for Amounts Included in Net Income									0.0	0.0	0.0
Foreign Currency Translation Adjustment									0.1	0.5	0.4
Gain on Cash Flow Hedge									0.6	0.0	0.0
Other Comprehensive Income Tax Benefit (Expense)									(6.9)	54.2	(47.7)
Other Comprehensive Income (Loss), Net of Tax									12.7	(98.4)	87.4
Total Comprehensive Income (Loss)									29.5	(12.7)	201.9
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income									16.8	85.7	114.5
Unrealized Holding Gains (Losses) Arising During the Year:
Securities Held by Subsidiaries									(2.6)	(150.4)	261.7
Securities Held by Parent									0.2	(1.5)	(2.0)
Reclassification Adjustment for Amounts Included in Net Income:
Securities Held by Subsidiaries									0.2	(26.9)	(26.8)
Securities Held by Parent									0.0	1.5	1.7
Unrealized Holding Gains (Losses)									(2.2)	(177.3)	234.6
Unrecognized Postretirement Benefit Costs Arising During the Year									14.2	3.0	(106.4)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax									1.0	0.0	0.0
Amortization of Unrecognized Postretirement Benefit Costs									5.3	23.1	7.9
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax									6.3	23.1	7.9
Net Unrecognized Postretirement Benefit Costs									20.5	26.1	(98.5)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, before Tax									(0.3)	(1.4)	(1.0)
Gain on Cash Flow Hedge									1.6	0.0	0.0
Other Comprehensive Income (Loss) Before Income Taxes									19.6	(152.6)	135.1
Income Tax Benefit (Expense):
Securities Held by Subsidiaries									1.1	53.4	(92.3)
Securities Held by Parent									(0.1)	0.5	0.7
Securities Held by Subsidiaries									(0.1)	9.4	9.4
Securities Held by Parent									0.0	(0.5)	(0.6)
Unrealized Holding Gains and Losses									0.9	62.8	(82.8)
Unrecognized Postretirement Benefit Costs Arising During the Year									(5.0)	(1.1)	37.5
Curtailment Cost Recognized									(0.4)	0.0	0.0
Amortization of Unrecognized Postretirement Benefit Costs									(1.9)	(8.0)	(2.8)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax									2.3	8.0	2.8
Net Unrecognized Postretirement Benefit Costs									(7.3)	(9.1)	34.7
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Held by Subsidiaries, Tax									0.1	0.5	0.4
Gain on Cash Flow Hedge									0.6	0.0	0.0
Other Comprehensive Income Tax Benefit (Expense)									(6.9)	54.2	(47.7)
Other Comprehensive Income (Loss), Net of Tax									12.7	(98.4)	87.4
Total Comprehensive Income (Loss)									$ 29.5	$ (12.7)	$ 201.9